Calorimetric and thermodielectrical measurements of water interactions with some food materials.

A new thermoanalytical method, which allows the measurement of complex dielectrical permittivities as a function of temperature in the microwave frequency domain, is described and compared to Differential Scanning Calorimetry (DSC) with respect to the characterization of water in food materials. Ice crystallization temperatures and melting enthalpies, measured by DSC dynamically on the same sample, allowed precise determination of the amount of frozen water and its enthalpy as a function of total water content, especially at low water contents near the unfrozen water limit. Thermal and Dielectrical Analysis (TDA) measurements provide immediate information about water interactions in food materials, even down to the lowest water contents, which are generally difficult to assess by other techniques. Dielectrical behaviour of eight glucose-water melts, containing from 0 to 24% water, has been examined as a function of temperature. The dependence of the observed dielectric relaxations on water content and temperature are discussed, and the results obtained by TDA are compared to those from conventional frequency sweeping determinations. The influence of temperature, hydration, and state of the material on dielectrical relaxation determinations are also discussed, with reference to glucose and sorbitol behaviour. The variations in dielectric constant during starch heating and dehydration are presented and analysed, with the aim of understanding the microwave drying process.